Holographic recording for recording information on a recording medium through the use of holography is typically performed by superimposing light that carries image information on reference light within the recording medium and by writing a resultingly generated interference pattern onto the recording medium. For reproducing the information recorded, the recording medium is irradiated with reference light so that the image information is reproduced through diffraction derived from the interference pattern.
Recently, volume holography, or digital volume holography in particular, has been developed and is attracting attention in practical fields for ultra-high density optical recording. Volume holography is a method for writing a three-dimensional interference pattern by making positive use of a recording medium in the direction of thickness as well, and is characterized in that an increase in thickness can enhance the diffraction efficiency and a greater recording capacity can be achieved by employing multiplex recording. Digital volume holography is a computer-oriented holographic recording method which uses the same recording medium and recording method as with the volume holography, whereas the image information to be recorded is limited to binary digital patterns. In the digital volume holography, analog image information such as a picture is once digitized and developed into two-dimensional digital pattern information, and then it is recorded as image information. For reproduction, this digital pattern information is read and decoded to restore the original image information for display. Consequently, even if the SN ratio (signal-to-noise ratio) in the reproduction is somewhat poor, differential detection and/or error correction on encoded binary data enable to reproduce the original information with extremely high fidelity.
In holographic recording through the use of volume holography, various methods of multiplex recording have been proposed to improve the recording density for achieving increased recording capacity. One of those methods is angular multiplexing that is to record information with angles of the reference light varied from information to information to be recorded. Another method is phase encoding multiplexing as described in ‘Heanue, J. F., et al., “Recall of linear combinations of stored data pages based on phase-code multiplexing in volume holography”, Optics Letters, Vol. 19, No. 14, pp. 1079-1081, 1994’ and ‘Heanue, J. F., et al., “Encrypted holographic data storage based on orthogonal-phase-code multiplexing” Applied Optics, Vol. 34, No. 26, pp. 6012-6015, 1995’.
For materials of a recording medium for holographic recording, generally used are materials whose optical characteristics such as refractive index, permittivity, and reflectivity change depending on intensity of light when irradiated with light. Therefore, to erase information recorded on the recording medium, it is necessary that the areas on which the information is recorded be uniformized in the optical characteristics.
In the case where the information is recorded on the recording medium by multiplex recording, however, according to an erasing method as described above, it is not possible to erase part of the information selectively, because is erased all of the information recorded on the areas that are thus uniformized in the optical characteristics.
Moreover, in conventional holographic recording, the changes in the optical characteristics of the recording medium are often nonreversible, which makes it difficult to rewrite information.